Java's seven major sortings: heap sort (selection sort)

(1) Basic idea

Heapsort refers to a sorting algorithm designed using a data structure such as a stacked tree (heap). It is a type of selection sort. It selects data through the heap. It should be noted that if you sort in ascending order, you need to build a large pile, and if you sort in descending order, you need to build a small pile.

(2) Code implementation

Step one: First build a large root heap, starting from the last subtree and adjusting downward.

Step 2: Swap the 0 subscript with the last element, and then continue adjusting downwards

 

 public static void heapSort(int[] array) {
        createBigHeap(array);
        int end = array.length-1;
        while (end > 0) {
            swap(array,0,end);
            shiftDown(array,0,end);
            end--;
        }

    }
    public static void createBigHeap(int[] array) {
         for (int parent = (array.length-1-1)/2; parent >= 0; parent--){
             shiftDown(array,parent, array.length);
        }
    }
    private static void shiftDown(int[] array,int parent,int len) {
        int child = 2*parent +1;
        while (child < len){
            if (child+1 < len && array[child] < array[child+1]) {
                child++;
            }
            if (array[child] > array[parent]) {
                swap(array,child,parent);
                parent = child;
                child = 2*parent +1;
            } else {
                break;
            }
        }
 }

(3) Feature summary

1. Heap sort uses a heap to select numbers, which is much more efficient.

2. Time complexity: O(N*logN); Space complexity: O(1)

3. Stability: unstable